A ball nose end mill is typically used to cut curved surfaces. It is divided into two classes depending on whether there is a cutting insert, i.e., a solid ball nose end mill in which a ball nose portion is integrated with an end mill body and an indexable ball nose end mill wherein a cutting insert is releasibly mounted on an end mill body.
In the indexable ball nose end mill, the cutting insert is securely mounted on the tool body through a screw and the like so that it can stably perform cutting tasks. In such a case, since the screw provides the insert with only a vertical clamping force, it cannot sufficiently support the reaction force caused by the cutting force. Particularly, in the cutting insert used for the ball nose end mill having a circular arc-shaped cutting edge for cutting the curved surface, the insert may rotate around the screw due to the reaction force caused by the cutting force. That is, there is a problem in that the cutting insert cannot be securely mounted on the tool body by using only the screw.
FIG. 1 illustrates a ball nose end mill disclosed in Korean Laid-Open Patent Publication No. 10-2002-83423 as an example of the prior art that modifies the mount structure of the cutting insert and the insert pocket in order to solve such a problem. Further, FIG. 2 schematically illustrates the cutting insert shown in FIG. 1.
Referring to FIGS. 1 and 2, a recess 23 formed at a bottom surface 22 of a cutting insert 2 engages with a protrusion 33 formed at a bottom wall 31 of an insert pocket 3. The cutting insert 2 is vertically fixed on the insert pocket 3 by a screw. The engagement between the protrusion 33 and the recess 23 secures the cutting insert 2 on the insert pocket 3 so that the cutting insert 2 cannot rotate.
In the prior art, the cutting insert needs the recess formed at the bottom surface. This is because the cutting insert without the recess may rotate around the screw on the insert pocket while performing cutting tasks. Thus, the prior ball nose end mill uses a single-sided cutting insert, which has cutting edges 25, 26 formed only on one surface of the cutting insert 2.
However, in the conventional single-sided cutting insert, while the first cutting edge 25 performs the cutting tasks, the second cutting edge 26 is inevitably exposed to the outside cutting environment. During the cutting tasks, chips generated by the first cutting edge contact the exposed second cutting edge, thereby causing the second cutting edge to wear out. Thus, there is a problem in that the second cutting edge of the cutting insert for the prior ball nose end mill wears out considerably before it is actually used for performing the cutting tasks. As such, its service life is significantly reduced.